MF-TDMA low earth orbit satellite time synchronization method based on feedback mechanism

Abstract

The invention discloses an MF-TDMA low earth orbit satellite time synchronization method based on a feedback mechanism, and the method comprises the steps: building a bidirectional link communication model, enabling a satellite to transmit a signal to a ground station terminal through a downlink, and enabling the satellite to receive a signal from the ground station terminal through an uplink; the ground station terminal synchronously establishes a ground time scale table through downlink acquisition, performs coarse time compensation according to the time delay, and sends an uplink signal according to the ground time scale table; the satellite receives an uplink signal, synchronizes time and establishes a time error function; and the ground station terminal takes the time error function as a feedback value, dynamically adjusts the ground time dial gauge, and carries out fine time compensation until the time synchronization error is eliminated, thereby completing fine time synchronization. According to the invention, time synchronization between a plurality of ground station terminals and a satellite can be supported at the same time, efficient utilization of spectrum resources is realized, resource allocation can be optimized in real time through a feedback mechanism, and the method has high real-time performance and accuracy.

Description

Technical Field

[0001] The present invention belongs to the technical field of satellite communication, and relates to a method for time synchronization of low-earth orbit satellites based on a feedback mechanism in MF-TDMA. Background Art

[0002] With the rapid development of satellite communication technology, low-earth orbit satellite communication technology has been widely used in fields such as data transmission, remote sensing monitoring, and navigation positioning. And due to the high-speed movement and complex environment of low-earth orbit satellites, their communication is affected by factors such as low signal-to-noise ratio and high dynamics. There is a large relative radial movement between the satellite payload and the ground terminal, and the time synchronization between the satellite and the terminal has become an urgent problem to be solved. The error of time synchronization may lead to chaos in data transmission, a decrease in positioning accuracy, and a reduction in network efficiency.

[0003] The orbital altitude and speed of low-earth orbit satellites change, causing the transmission delay between the satellite and the ground terminal to change continuously, increasing the difficulty of time synchronization. Some of these high-precision time synchronization algorithms may have high complexity and require a large amount of computing resources and time. This may result in the inability to achieve real-time synchronization in practical applications, or the need for additional hardware support to accelerate the execution of the algorithm.

[0004] The multi-frequency time division multiple access (MF-TDMA) system emerged based on the requirements of the wireless communication field for efficient spectrum utilization and multi-user access. MF-TDMA is a hybrid multiple access technology that combines time division multiple access (TDMA) and frequency division multiple access (FDMA). By dividing time into multiple time slots and frequency into multiple frequency points, it allows multiple users to use the allocated frequency resources in different time slots, thereby achieving efficient multi-user communication. In satellite communication, due to the large signal propagation delay, the difficulty of time synchronization is further increased. A synchronization algorithm using a feedback mechanism is adopted to dynamically adjust the time slot allocation and clock deviation. The core idea of the feedback mechanism is to dynamically adjust the clock frequency and phase by real-time monitoring of the clock state, signal propagation delay, and network topology changes, so as to achieve real-time compensation for time errors.

[0005] Common ones include ranging methods, pseudo-code high-precision distance measurement methods, and two-way time comparison methods, etc. These methods achieve satellite time synchronization to a certain extent, but there are still deficiencies. Among them, the ranging method is limited by factors such as atmospheric delay and multipath effects; the pseudo-code high-precision distance measurement method relies on more complex signal processing and hardware system support; the two-way time comparison method has very high requirements for the close cooperation between the satellite and the ground station. For low-earth orbit satellite communication in a high-dynamic environment, it is particularly important to develop an efficient and accurate low-earth orbit satellite time synchronization method. Summary of the Invention

[0006] The present invention aims to solve the technical problems of difficult and large-error real-time synchronization between satellites and ground terminals. The present invention provides a MF-TDMA low-earth orbit satellite time synchronization method based on a feedback mechanism, and the technical solution adopted is as follows:

[0007] A MF-TDMA low-earth orbit satellite time synchronization method based on a feedback mechanism, comprising the steps of:

[0008] S1. Establish a two-way link communication model, where the two-way link communication model includes a downlink and an uplink. The satellite sends signals to the ground station terminal through the downlink and receives signals from the ground station terminal through the uplink;

[0009] S2. The ground station terminal establishes a ground time scale through downlink acquisition synchronization;

[0010] S3. The ground station terminal performs rough time compensation according to the time delay;

[0011] S4. The ground station terminal sends an uplink signal according to the ground time scale. The satellite receives the uplink signal, and at the synchronization moment, a time error function is established;

[0012] S5. The ground station terminal uses the time error function as a feedback value to dynamically adjust the ground time scale and perform fine time compensation until the time synchronization error is eliminated and fine time synchronization is completed.

[0013] In an embodiment of the present invention, the S1 includes:

[0014] S11. Design the signal frame format of the downlink. The data in the synchronization time slot is a pseudo-random sequence and a time stamp, and the length of the synchronization time slot is T 1 , there are n pieces of service information in total, that is, there are n service time slots, and the length of the service time slot is T 2 , then the time length of one frame of data is T 1 +nT 2 ;

[0015] S12. Design the signal frame format of the uplink. The data in the synchronization header carries ID address information, and the length of the time slot is Tu 1 , there are m pieces of service information in total, that is, there are m service time slots, and the length of the time slot is Tu 2 , then the time length of one frame of data is Tu 1 +mTu 2 .

[0016] In an embodiment of the present invention, the S2 includes:

[0017] S21. Establish a time scale t based on the clock on the satellite s, a scale time is Δt, with the dimension of the microsecond level. The time of one frame of data is l times of Δt. The satellite continuously transmits downlink signals at integer multiples of lΔt. That is, each timestamp information is an integer multiple of lΔt;

[0018] S22. The ground station terminal calculates the preliminary estimate of the Doppler frequency offset by calculating the real-time position and speed of the satellite, performs coarse frequency offset compensation, demodulates the downlink signal after down-conversion processing, correlates it with the local sequence, and performs frequency offset estimation. According to the estimated value, fine frequency offset compensation is performed to achieve downlink capture synchronization;

[0019] S23. Establish the ground time scale table.

[0020] In an embodiment of the present invention, the S23 includes:

[0021] S231. The ground station terminal performs CRC check on the timestamp information, and judges whether the received downlink signal conforms to the satellite motion law or is interfered by a complex electromagnetic environment according to the change rate of the historical frequency offset estimation value and the signal quality factor;

[0022] S232. The ground station terminal sets the ground time scale table t g , and accumulates time uniformly with the Δt time scale, and performs periodic maintenance and update with the timestamp information that has passed the full check.

[0023] In an embodiment of the present invention, the S3 includes:

[0024] Perform coarse time compensation according to the satellite-ground delay and the local hardware and software processing delay;

[0025] The ground station terminal combines the ephemeris to estimate the satellite-ground delay and performs coarse time compensation, and the compensation amount is t coarse ;

[0026] The ground station terminal tests and calculates the inherent time offset brought by the hardware and software of the ground system, and the compensation amount is t co .

[0027] In an embodiment of the present invention, the S4 includes:

[0028] S41. The ground station terminal sends the uplink signal according to the ground time scale table, and the satellite receives the uplink signal;

[0029] S42. The satellite calculates the time error function for the ground station terminal according to the ID address decoded from the uplink synchronization header, and configures the time error function in the first service information time slot, that is, the status time slot, of the downlink signal.

[0030] In one embodiment of the present invention, the representation method of the time error function is as follows:

[0031] Given the ground station terminal time scale t g , and the accurate time scale t s of the satellite terminal is only recorded on the satellite. Using the master-slave time synchronization method with the satellite as the master, the clocks of each ground station terminal are in the state of tracking the reference clock. When the satellite receives the uplink signal sent by the ground station terminal, the synchronization moment is t', and the expected synchronization moment of the satellite is t e , a time error function e is established, and the expression is: e = t' - t e .

[0032] In one embodiment of the present invention, S4 further includes:

[0033] S43. By analyzing the change rate of the time error function of each ID address, the channel quality is evaluated, and the frequency and time slot allocation are dynamically adjusted to avoid severely interfered frequency bands.

[0034] In one embodiment of the present invention, S5 includes:

[0035] The ground station terminal solves the corresponding time error function according to the ID address included in the status time slot. If the corresponding time error function cannot be solved, it means that the ground station terminal has not achieved uplink synchronization. At this time, a search-type adjustment of the time compensation amount is required until uplink synchronization is achieved, and the accurate time compensation is realized with the time error function as the feedback value.

[0036] In one embodiment of the present invention, the time compensation amount is expressed as: ΔT = t coarse + t co - e, where t coarse is the rough time compensation amount of the satellite-ground delay, t co is the compensation amount of the hardware and software, and e is the time error function.

[0037] Advantages of the present invention:

[0038] 1. The MF-TDMA low-earth orbit satellite time synchronization method based on the feedback mechanism of the present invention establishes a time error function between the synchronization moment of the uplink and the expected synchronization moment set by the satellite's local time reference, and thus builds a feedback mechanism. The ground station terminal can dynamically adjust the time compensation amount according to the feedback value to track the satellite's time. Even if uplink out-of-step occurs at a certain beat, the ground station terminal can calculate the satellite-ground delay and scan and adjust the time compensation amount within a reasonable time interval to achieve rough uplink synchronization, and then achieve fine synchronization with the feedback mechanism, which has high real-time performance and accuracy and is easy to implement.

[0039] 2. The MF-TDMA low-earth orbit satellite time synchronization method based on a feedback mechanism of the present invention divides the frequency band into multiple sub-bands (FDMA) by the MF-TDMA system, and further divides time slots (TDMA) within each sub-band, achieving efficient utilization of spectrum resources. Moreover, it can optimize resource allocation in real time through the feedback mechanism to ensure communication quality, that is, by analyzing the change rate of the time error function of each ID address, the channel quality is evaluated, and the frequency and time slot allocation are dynamically adjusted to avoid severely interfered frequency bands. Compared with traditional time synchronization methods, it can support time synchronization between multiple ground station terminals and satellites simultaneously. In the case of a harsh channel, better time synchronization can be achieved through the allocation of time slot resources and frequency domain resources. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 is a flowchart of the MF-TDMA low-earth orbit satellite time synchronization method based on a feedback mechanism provided by an embodiment of the present invention;

[0041] Figure 2 is a schematic diagram of one satellite and multiple locations provided by an embodiment of the present invention;

[0042] Figure 3 is a schematic diagram of the signal frame format of the downlink provided by an embodiment of the present invention;

[0043] Figure 4 is a schematic diagram of the signal frame format of the uplink provided by an embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0044] The present invention will be described in detail below with reference to the drawings and specific embodiments.

[0045] The present invention provides an MF-TDMA low-earth orbit satellite time synchronization method based on a feedback mechanism, and builds a two-way stable communication link, which has high bandwidth, low latency and anti-interference capabilities to ensure the transmission and reception of synchronization signals. Based on the two-way link, a feedback mechanism is designed and error processing, that is, time compensation, is performed to finally achieve accurate time synchronization.

[0046] Referring to the attached Figure 1 , the MF-TDMA low-earth orbit satellite time synchronization method based on a feedback mechanism includes the following steps:

[0047] S1. Establish a two-way link communication model. The two-way link communication model includes a downlink and an uplink. The satellite sends signals to the ground station terminal through the downlink and receives signals from the ground station terminal through the uplink;

[0048] S2. The ground station terminal establishes a ground time scale through downlink capture synchronization;

[0049] S3, the ground station terminal performs coarse time compensation according to the time delay;

[0050] S4, the ground station terminal sends an uplink signal according to the ground time scale table, the satellite receives the uplink signal, synchronizes the time, and establishes a time error function;

[0051] S5. The ground station terminal uses the time error function as the feedback value to dynamically adjust the ground time scale table and perform precise time compensation until the time synchronization error is eliminated and precise time synchronization is completed.

[0052] ID addresses are assigned to ground station terminals. A satellite can establish link communications with multiple ground terminal stations, and multiple access technology is used to distinguish signals from different ground stations. The uplink signal has multiple frequencies to choose to send, and there are multiple time slots to configure business information. It can be sent in all time slots or in a single time slot.

[0053] In the present invention, S1 includes a downlink design and an uplink design.

[0054] S11. Design the signal frame format of the downlink. The data of the synchronization time slot is a pseudo-random sequence and a timestamp. The length of the synchronization time slot is T 1 , there are n business information in total, that is, there are n business time slots in total, and the length of the business time slot is T 2 , then the time length of a frame of data is T 1 +nT 2 .

[0055] To send or receive a frame of data, you need to know the length to separate the data of each time slot. Capturing synchronization will find the time point of the synchronization time slot, and then calculate the time length to know which time slot the data corresponds to. In order to carry more information, business information uses a higher symbol rate. And x frames form a superframe for sending data.

[0056] S12. Design the signal frame format of the uplink. The data of the synchronization header carries the ID address information. The time slot length is Tu 1 There are m business information in total, that is, there are m business time slots, and the time slot length is Tu 2 , then the time length of a frame of data is Tu 1 +mTu 2 . And x frames form a superframe for sending data.

[0057] There are multiple carrier modes for upconversion to achieve frequency division multiple access. Time division multiple access is reflected by the frame format, where different time slots have different data and are separated in time. Both the downlink signal and the uplink signal use time division multiple access. The downlink is a signal sent by the satellite, and each terminal receives the same signal. The data within different service time slots are allocated to different terminals. Frequency division multiple access means that the uplink signals sent by different terminals are at different frequency points, which allows multiple terminals to send uplink signals to the satellite simultaneously. If they are all at the same frequency point, the uplink signals will conflict.

[0058] For the downlink frame, each frame has synchronization time slot data, while the uplink only has a synchronization header in the first frame of the superframe. The purpose is to reduce the processing pressure of the satellite for uplink synchronization because the satellite needs to process uplink signals sent by multiple terminals.

[0059] In an embodiment of the present invention, S2 includes:

[0060] S21. The space-ground system is powered on, and a time scale table t is established based on the clock on the satellite. s One scale time is Δt, with a dimension in the microsecond level. The time of one frame of data is l times Δt. The satellite continuously transmits downlink signals at integer multiples of lΔt, that is, each timestamp information is an integer multiple of lΔt.

[0061] S22. The ground station terminal calculates the Doppler frequency offset through the real-time position and speed of the satellite over a period of time, and then obtains a preliminary estimate of the frequency offset for coarse frequency offset compensation. After demodulating the downlink signal processed by downconversion and correlating it with the local sequence, frequency offset estimation is performed, and then fine frequency offset compensation is carried out according to the estimation value to achieve downlink acquisition synchronization. Relying on the pseudo-random sequence in the synchronization time slot, the signal is correlated with the local pseudo-random sequence, and it is detected whether the correlation peak is greater than the set decision threshold to achieve acquisition synchronization.

[0062] Relying on the pseudo-random sequence in the synchronization time slot. The signal is correlated with the local pseudo-random sequence, and it is detected whether the correlation peak is greater than the set decision threshold to achieve acquisition synchronization. A carrier synchronization algorithm PMF-FFT (Piecewise Matched Filter - Fast Fourier Transform combination) is used to estimate the frequency offset.

[0063] Finally, the timestamp information and the service information are successfully differentially demodulated. The timestamp signal is differentially BPSK (Binary Phase Shift Keying) modulated, and the rest are differentially QPSK (Quadrature Phase Shift Keying) modulated.

[0064] S23. Establish a ground time scale table. To avoid errors in timestamp information that directly affect system operation, the ground station terminal uses CRC (cyclic redundancy check) to check the timestamp information, and judges whether the received downlink signal conforms to the satellite motion law or is interfered by a complex electromagnetic environment based on the rate of change of the historical frequency deviation estimate and the signal quality factor. The satellite's telemotion is from far to near and then from near to far for the ground. Low-orbit satellites revolve around the earth, and the frequency deviation changes linearly. If the difference between two consecutive frequency deviation estimates is abnormal, then this section of data may be abnormal, and it can be judged that the downlink signal is interfered with. For example, on rainy days, the communication is poor, and the conversion rate may fluctuate greatly.

[0065] The specific method of the ground station terminal to verify the timestamp information is as follows: the sender divides the original data by the generating polynomial modulo 2, obtains the remainder (i.e., the CRC check code), attaches it to the original data, and then sends it. The receiver uses the same generating polynomial to receive the data and performs modulo 2 division. If the remainder is 0, the data is correct.

[0066] Therefore, the ground station terminal sets the ground time scale table t g , the accumulated time is uniformly calculated in Δt time scale, and the fully verified timestamp information is periodically maintained and updated. Therefore, t g The difference in actual 1 second on the ground is dynamically scaled.

[0067] In one embodiment of the present invention, S3 includes: performing coarse time compensation according to the satellite-ground delay and the local hardware and software processing delay. The ground station terminal estimates the satellite-ground delay in combination with the ephemeris and performs coarse time compensation, and the compensation amount is t coarse ; The ground station terminal tests and calculates the inherent time deviation caused by the hardware and software of the ground system, and the compensation amount is t co .

[0068] In one embodiment of the present invention, S4 includes:

[0069] S41. The ground station terminal sends an uplink signal according to the ground time scale, and the satellite receives the uplink signal.

[0070] After coarse time compensation, the uplink synchronization probe can be sent according to the local time scale. Compared with the downlink, each frame has synchronization information. The uplink has a synchronization header every x frames, and the synchronization header time slots of the remaining frames are filled with 0. Assuming that the time error function is 0, the synchronization probe is sent at every integer multiple of xlΔt. However, in the actual environment, due to factors such as satellite-to-ground delay and satellite movement, it is necessary to adjust the uplink transmission to advance or delay the transmission according to the time compensation amount. When delaying the transmission, only the next x frames of data need to be delayed as a whole, which will not affect the previous frame of data. If it is sent in advance, the previous frame of data needs to be truncated.

[0071] S42. After the satellite is uplink synchronized, it will calculate the time error function for the corresponding ground station terminal based on the ID address decoded from the uplink synchronization header. And the time error function is configured in the first service information time slot of the downlink signal, that is, the status time slot. One ID address is followed by one time error function value, and multiple ID addresses are included in one status time slot.

[0072] Given the time scale table t of the ground station terminal g , and the accurate time scale table t of the satellite terminal s is only recorded on the satellite. Using the master-slave time synchronization method with the satellite as the master, the clocks of each ground station terminal are in the state of tracking the reference clock. When the satellite receives the uplink signal sent by the ground station terminal, the synchronization moment is t', and the expected synchronization moment of the satellite is t e , a time error function e is established, and the expression is: e = t' - t e . For e, a positive value indicates that the uplink signal sent by the ground terminal station lags, and a negative value indicates that the uplink signal sent by the ground terminal station is ahead.

[0073] S43. By analyzing the change rate of the time error function for each ID address, the channel quality is evaluated, and the frequency and time slot allocation are dynamically adjusted to avoid severely interfered frequency bands.

[0074] The ground station terminal also decodes the corresponding time error function according to the ID address included in the status information. If the corresponding time error function cannot be decoded, it means that the ground station terminal has not achieved uplink synchronization. At this time, a search-type adjustment needs to be made to the time compensation amount within a reasonable range until uplink synchronization is achieved, and the time error function is used as the feedback value to achieve precise time compensation.

[0075] The length of the superframe is fixed. The time compensation amount will cause the frame to move in time. If the movement amount is too large, it will exceed the range, and the superframe will move to a position beyond the previous superframe or beyond the next superframe.

[0076] When the satellite receives the uplink signals sent by each terminal, it will perform synchronization. The satellite will set a standard synchronization time range. Within this range, synchronization operations will be performed. Generally, the time length of one superframe is set. If the synchronization moment t' exceeds the range, uplink synchronization is not considered. Only when uplink synchronization is considered successful will there be a time error function configuration. If the ground terminal sends an uplink signal, but the time error function cannot be decoded in the status time slot of the downlink, it means that there is no synchronization with the satellite uplink, that is, out of sync.

[0077] The expression of the time compensation amount is: ΔT = t coarse + t co - e, where t coarseis the coarse time compensation amount for the satellite-ground time delay, t co is the compensation amount of hardware and software, and e is the time error function. For ΔT, t coarse 、t co , a positive time compensation amount indicates an adjustment amount for time delay, and a negative value indicates an adjustment amount for time advance.

[0078] A complete feedback mechanism is thus established. The ground station terminal continuously iteratively adjusts the time compensation amount in a tracking manner, and finally achieves a state where the time error function tends to 0, that is, the time error function is less than the set value of the accuracy requirement, which is regarded as successfully completing the precise time synchronization. The uplink signal can be sent on different frequency points and can also be sent in different time slots. The present invention optimizes resource allocation in real time through the feedback mechanism to ensure communication quality. The change rate of the time error function of each ID address at the satellite end fluctuates smoothly, indicating good channel quality, and if the fluctuation is large, the channel quality is relatively poor.

[0079] In summary, the MF-TDMA low-earth orbit satellite time synchronization method based on the feedback mechanism of the present invention establishes a two-way link communication model and designs an MF-TDMA system, which is reflected in the frame formats of the downlink and uplink. The time clock of the satellite is the benchmark for the entire satellite-ground system. The downlink signal is sent from the satellite to each ground station terminal. The ground station terminal obtains the timestamp information through acquisition and synchronization, establishes its own local time scale, performs coarse time synchronization according to the satellite-ground time delay and the local hardware and software processing time delay, and then sends the uplink signal corresponding to the ID. The satellite receives the uplink signal, and after synchronization evaluation, sends the feedback value. The ground station terminal continuously adjusts its local clock dynamically with the feedback value to track the satellite time clock, and finally eliminates the time synchronization error to achieve precise time synchronization.

[0080] Different from the traditional time synchronization that requires two-way time synchronization adjustment, the present invention patent is a master-slave time synchronization method with a clear architecture. The satellite is responsible for generating and distributing the time reference signal. The ground station terminal only needs to receive and adjust its local clock, and then send the uplink synchronization signal for the satellite to evaluate the corresponding ground station terminal and generate the time error function, without adjusting the satellite time clock. And the satellite, as the center of the network, can efficiently manage and synchronize the time of all ground station terminals to ensure the time consistency of the entire network. There is no need to adjust the time signal in a high-precision mode according to the motion state of the satellite and the Doppler effect, and only need to ensure a certain accuracy for the time signal adjustment, and the effective improvement of the accuracy is realized through the assistance of the feedback mechanism, which greatly enhances the adaptability of low-earth orbit satellite communication to the high-dynamic change environment.

[0081] As described above, it is only the specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be covered by the protection scope of the present invention.

Claims

1. A MF-TDMA low-orbit satellite time synchronization method based on a feedback mechanism, characterized in that: Includes steps: S1. Establishing a two-way link communication model, wherein the two-way link communication model includes a downlink and an uplink, wherein the satellite sends a signal to a ground station terminal through the downlink and receives a signal from the ground station terminal through the uplink; S2, the ground station terminal establishes a ground time scale table through downlink capture synchronization; S3, the ground station terminal performs coarse time compensation according to the time delay; S4, the ground station terminal sends an uplink signal according to the ground time scale table, the satellite receives the uplink signal, synchronizes the time, and establishes a time error function; S5. The ground station terminal uses the time error function as a feedback value to dynamically adjust the ground time scale table and perform precise time compensation until the time synchronization error is eliminated and precise time synchronization is completed.

2. The MF-TDMA low-orbit satellite time synchronization method based on a feedback mechanism according to claim 1, characterized in that: The S1 includes: S11, design the signal frame format of the downlink, the data of the synchronization time slot is a pseudo-random sequence and a timestamp, the length of the synchronization time slot is T1, there are n business information in total, that is, there are n business time slots in total, the length of the business time slot is T2, and the time length of one frame of data is T1+nT2; S12. Design the signal frame format of the uplink. The data of the synchronization header carries the ID address information. The time slot length is Tu1. There are m business information in total, that is, there are m business time slots. The time slot length is Tu2. Then the time length of one frame of data is Tu1+mTu2.

3. The MF-TDMA low-orbit satellite time synchronization method based on feedback mechanism according to claim 1, characterized in that: The S2 includes: S21. Establish a time scale table based on the clock on the satellite s , one tick time is Δt, dimension is microsecond level, one frame of data time is l times of Δt, the satellite transmits downlink signal uninterruptedly at the time of integer multiple of lΔt, that is, each timestamp information is an integer multiple of lΔt; S22, the ground station terminal obtains a preliminary estimate of the Doppler frequency offset by calculating the real-time position and speed of the satellite, performs coarse frequency offset compensation, demodulates the downlink signal processed by down-conversion, correlates it with the local sequence, and estimates the frequency offset, performs fine frequency offset compensation based on the estimated value, and realizes downlink capture synchronization; S23, establishing the ground time scale table.

4. The MF-TDMA low-orbit satellite time synchronization method based on feedback mechanism according to claim 3 is characterized in that: The S23 includes: S231, the ground station terminal uses CRC check on the timestamp information, and determines whether the received downlink signal complies with the satellite motion law or is interfered by a complex electromagnetic environment according to the change rate of the historical frequency offset estimation value and the signal quality factor; S232, the ground station terminal sets the ground time scale table t g , the accumulated timing is uniformly calculated using the Δt time scale, and periodic maintenance and updates are performed using the fully verified timestamp information.

5. The MF-TDMA low-orbit satellite time synchronization method based on feedback mechanism according to claim 1, characterized in that: The S3 includes: Implement coarse time compensation based on satellite-ground delay and local hardware and software processing delay; The ground station terminal estimates the satellite-ground delay based on the ephemeris and performs coarse time compensation. The compensation amount is t coarse ; The ground station terminal tests and calculates the inherent time deviation caused by the hardware and software of the ground system, and the compensation amount is t co .

6. The MF-TDMA low-orbit satellite time synchronization method based on feedback mechanism according to claim 1, characterized in that: The S4 includes: S41, the ground station terminal sends an uplink signal according to the ground time scale table, and the satellite receives the uplink signal; S42. The satellite calculates a time error function for the ground station terminal according to the ID address decoded from the uplink synchronization header, and configures the time error function in the first service information time slot of the downlink signal, namely, the status time slot.

7. The MF-TDMA low-orbit satellite time synchronization method based on feedback mechanism according to claim 6, characterized in that: The time error function is expressed as: It is known that the ground station terminal time scale table t g , satellite terminal accurate time scale t s Only recorded in the satellite, using master-slave time synchronization, with the satellite as the main, the clocks of each ground station terminal are in the state of tracking the reference clock, the satellite receives the uplink signal sent by the ground station terminal, the synchronization time is t', and the satellite expects the synchronization time to be t e , establish the time error function e, the expression is: e = t'-t e .

8. The MF-TDMA low-orbit satellite time synchronization method based on feedback mechanism according to claim 7, characterized in that: The S4 further comprises: S43. By analyzing the change rate of the time error function of each ID address, the channel quality is evaluated, and the frequency and time slot allocation are dynamically adjusted to avoid the frequency band with serious interference.

9. The MF-TDMA low-orbit satellite time synchronization method based on feedback mechanism according to claim 1, characterized in that: The S5 includes: The ground station terminal solves the corresponding time error function according to the ID address contained in the status time slot. If the corresponding time error function cannot be solved, it means that the ground station terminal has not achieved uplink synchronization. At this time, it is necessary to perform a search-based adjustment on the time compensation amount until uplink synchronization is achieved, and use the time error function as a feedback value to achieve precise time compensation.

10. The MF-TDMA low-orbit satellite time synchronization method based on feedback mechanism according to claim 9, characterized in that: The time compensation amount is expressed as: ΔT = t coarse +t co -e, where t coarse is the coarse time compensation of the satellite-to-ground delay, t co is the compensation amount of hardware and software, and e is the time error function.

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